Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Why you’ll always need a telescope
bigger & better than that: nothing
ever works as expected
This Talk
How to empirically determine the minimum aperture size needed to resolve closely-spaced objects & fine details
The Experiment
TODAY’S LESSONS
• Refractors use lenses– Problematic, no
longer used for research
• Reflectors use mirrors–Most modern
telescopes are reflectors
REFRACTORS VS. REFLECTORS
REFRACTOR ISSUES:CHROMATIC ABERRATION
Lens’ focal
length varies
with wave-length of light being focused
– violet light
refracts most,
red least
This fix
works for
hobbyists,
but results are not
research-
grade
Yerkes Telescope (U of Chicago), the world’s largest refractor:
• 63 ft (19.2 m) long
• Tube alone is 6 tons –sagging is a problem
• 90 ft (27 m) high dome
• Only 40 in (1.02 m) in diameter!
REFRACTOR ISSUES:
SIZE
REFLECTORS ARE ADAPTABLE (& COMPACTABLE)
Telescopes at the CTO use the Cassegrain focus.
• Spherical optics (lenses & mirrors) are easy & cheap to make - HOWEVER - focal length varies with distance from center of optic
• Solution 1 ($$$): use a parabolic optic
• Solution 2 ($): use a corrective lens make a Schmidt Cassegrain Telescope
SPHERICAL ABERRATION
• Parabolic optics focus all incoming light to 1 point no matter where it hits the optic, BUT ONLY IF all rays enter to optic’s center
– Otherwise, focal length depends on proximity to edge
• Ideal Solution ($$$): use hyperbolic optics (like parabolic but a little more open) make a Ritchey-Chrétien (RC) telescope
– Nearly all major research observatories use RC reflector telescopes
• Alternative: more corrective lens
Bottom line – nothing ever works [perfectly].
COMA
ASTRONOMICAL SEEING (WHY STARS TWINKLE)
Actual
star size: ⦁
Bigger aperture less diffraction smaller Airy patterns better resolution
RESOLVING POWER RECAP
MAGNIFICATION
• Ratio of the focal lengths of
the 2 optics
• Magnification is unitless
• Unlike aperture size, greater
magnification doesn’t really
improve image quality
ANGULAR MEASUREMENTS RECAP
The Sun & full Moon
are both about 0.5°or 30’ across
• D = aperture diameter
• h = linear separation of objects in the image (same units as ℓ)
• θ = angular separation
• ℓ = distance to the target (same units as h)
DAWES’ LAW
1. Measure distance ℓ from target to aperture
– Linear separation h is given in the manual.
2. Measure the aperture diameter, D, when the image (either stars or “canals”) just becomes unresolved (the 2 objects appear to start touching). One person should adjust the telescope iris, one should measure the aperture, & one should record the data point.
– Don't bump the telescope or you'll have to remeasure ℓ!
– Don't let your ruler touch the iris diaphragm—the metal leaves are easily dislodged & time-consuming to fix!
3. Repeat step 2 until you have 5 trials each for the binary star AND the “canals”
PROCEDURES (1/2)
4. Average your results from the previous step to obtain 2 experimental aperture sizes—1 each for resolving the stars & the “canals”.
5. Calculate the Dawes’ Limit (D) using your previously-measured values for distance ℓ & linear separation h; these are your predicted aperture sizes needed to resolve the stars or “canals.”
6. Calculate the % error between your averaged experimental values & the predicted values you calculated using Dawes’ Limit.
7. List 3+ sources of error (uncertainty) in this experiment (remember your dos & don’ts!)
PROCEDURES (2/2)
• Read “The Astronomical Telescope II” for next class
• NO night lab next Monday (we don’t want to keep you from seeing astronaut Scott Kelly!)
Reminders:
• Use the worksheets from the lab manual for this lab, pages 3.5 to 3.8.
• Answer all questions, show work, & round answers to the correct significant digits.
• Keep your worksheets in a safe place so you can refer to them for your double formal lab report.
ONE MORE THING!